The invention relates to an apparatus for decoding a channel signal into an information signal, the channel signal comprising sequential blocks of information, a block of information comprising a sync word and a plurality of channel words, the information signal comprising sequential information words, the apparatus comprising
input means for receiving the channel signal, PA0 conversion means for converting the channel words into the information words, PA0 output means for supplying the information signal. The invention also relates to an information reproducing arrangement which includes said decoding apparatus. A decoding apparatus as defined in the opening paragraph can be used in a reproducing arrangement for reproducing information from an optical or an magnetic record carrier or in a receiver for receiving information from a broadcast transmission channel. An apparatus of the type as defined above is known from U.S. Pat. No. 5,148,330, U.S. Pat. No. 5,243,470, and U.S. Pat. No. 5,371,634. PA0 (i) an erasure indicating that the m-bit channel word is not a valid channel word, PA0 (ii) an n-bit information word.
The conversion means may be in the form of an m-to-n decoder, in which m-bit channel words are converted into n-bit information words, where n and m are integers such that m is larger than n.
The decoding apparatus includes error concealment means which can comprise an error detection and correction unit, so that the locations of errors in the information words supplied by the conversion means can be established and the erroneous information words be corrected. Or, simply error concealment can be carried out, such as by muting, without any correction, or both.
During reception, errors can occur in the channel words received. If the number of errors appear to be too large, it is assumed that the decoder is not synchronized anymore, so that a resynchronization is started by detecting the sync words included in the channel signal. The sync word in a channel signal is generally a unique word, and its bitsequence is thus different from the channel words occurring in the channel signal.
The invention aims at providing an improved decoding apparatus. The apparatus in accordance with the invention is characterized in that the conversion means comprises channel word detection means for establishing whether channel words received do not belong to a group of valid channel words, the channel word detection means being adapted to supply a detection signal upon such detection.
The invention is based on the following recognition. Let us assume that the decoder apparatus is provided with conversion means for converting m-bit channel words into n-bit information words. Generally, there are 2.sup.n information words, so that for n=8, 256 information words exist. For m=10, there are 2.sup.10 or 1020 possible channel words. For the conversion of the 256 information words, not all of the 1024 channel words are needed. One could argue that only 256 channel words are required, so that 1024-256 channel words are `not real` channel words, or invalid channel words.
The apparatus, when synchronized, is able to locate the m-bit channel words in the channel signal received and can successfully decode them into n-bit information words.
It is not desirable that the decoder apparatus keeps searching for the sync word all the time in the channel signal, as errors in the channel signal can generate a false sync pattern detection, which results in a loss of synchronization. Hence, the decoder apparatus should only search for sync words when it is not synchronized (out of lock).
If the decoder apparatus is out of lock, it is not able to find correctly the boundaries of the channel words in the channel signal. In this situation, the decoder will give two sort of results:
In an ideal situation for synchronization, a mis-synchronization would mean that the decoder apparatus would only be able to find erasures (invalid channel words), hence, a sequence of invalid channel words would certainly mean a mis-synchronization. However, channel codes are usually not specifically designed for implementation of mis-synchronization detectors.
There is a probability P that a decoder apparatus that is out of lock will detect an erasure. In general it can be stated that, for larger bitshifts, the probability will be higher.
In accordance with the invention, an early indication of an out of lock situation can be obtained by detecting invalid channel words. In dependence on the number of invalid channel words detected in a specified time interval, it is decided whether a mis-synchronization situation is present or not. More specifically, the number of invalid channel words present in the last F channel words received is counted. If the number exceeds a specified threshold value (K), it is decided that a mis-synchronization situation exists, so that the sync word detector can be enabled to search for a sync word.
The choice of the adequate values for F and K for the detection of a mis-synchronization situation is a function of the probability P and of the bit error rate SER (being the symbol error rate considering the decoder apparatus being in synchronization). F and K should be large enough to guarantee that the probability of having a detection caused by substitution error is very small.